图1 (a)地震波速层析成像显示西太平洋俯冲板片在地幔过渡带滞留形成的大地幔楔结构(据文献[1]修改);(b)东亚大地幔楔两个深部碳循环圈模型卡通图(据文献[2]修改)

Fig.1 (a) Seismic wave velocity tomography showing the big mantle wedge structure formed by the stagnation of the Western Pacific slab in the mantle transition zone (modified after [1]), and (b) cartoon model showing two modes of deep carbon cycling in the big mantle wedge in East Asia (modified after [2])

图2 中国东部晚白垩世和新生代板内玄武岩的δ26Mg值与MgO(a,c)、Cr(b,d)含量关系图(样品数据点引自文献[4],本文给出Cr尖晶石或橄榄石+Cr尖晶石的分离结晶模拟计算曲线) 解释详见正文。不同颜色的实线和虚线分别代表按不同矿物比例计算的分离结晶和堆晶模拟线。每条模拟线中的方块代表5%的增量,每个菱形代表1%的增量。橄榄石与熔体间的分配系数:D(Mg)=5.89[14],D(Cr)=0.85[15]。尖晶石与熔体间的分配系数:D(Mg)=1.8[14],D(Cr)≈760[14]。矿物与熔体间的Mg同位素分馏系数:Δ26Mg橄榄石-熔体≈0‰,Δ26Mg铬铁矿-熔体=1.18‰[4]。实心黑色圆圈(a, b)是为拟合数据假设的初始熔体MgO含量(约17%)和Cr含量(800×10-6),灰色圆圈(c, d)为文献常用的初始玄武岩熔体MgO含量(约11%)和Cr含量(700×10-6)[16]。

Fig.2 Plots of δ26Mg vs. MgO (a, c) and Cr (b, d) for Late Cretaceous and Cenozoic intraplate basalts from eastern China. Data are adapted from [4]. The calculated fractional crystallization lines for spinel or spinel + olivine are from this study. The solid and dashed lines with different colors represent the modeling results for fractional crystallization and accumulation with different mineral proportions, respectively. Squares and diamonds in each line represent 5% and 1% increments, respectively. The Mg and Cr partition coefficients between olivine and melt are 5.89 (after [14]) and 0.85 (after [15]), respectively, and between spinel and melt are 1.8 (after [14]) and 760 (after [14]), respectively. The Δ26Mgolivine-melt and Δ26Mgchromite-melt values are ~0‰ and 1.18‰, respectively (after [4]). Solid black circles in (a, b) are the assumed initial melt (MgO 17% and Cr 800×10-6, mass fraction) in this study, and gray circles in (c, d) are the initial basaltic melt (MgO 11% and Cr 700×10-6, mass fraction) commonly used in the literature (after [16]).

图3 δ26Mg与MgO(a)和Cr(b)的三端员混合的示意图。模拟计算的端员中心值:俯冲富镁碳酸盐w(MgO)=13%,w(Cr)=10×10-6,δ26Mg=-2.0‰[12];地幔橄榄岩w(MgO)=45%,w(Cr)=2 200×10-6,δ26Mg=-0.25‰[20-21];俯冲洋壳熔融的富硅熔体w(MgO)=2.5%,w(Cr)=26×10-6,δ26Mg=-0.25‰[22]。 (中国东部晚白垩世和新生代板内玄武岩数据据文献[4],混合线为本文计算)

Fig.3 Three end-member mixing plots. (a) δ26Mg vs. MgO. (b) δ26Mg vs. Cr. Data are adapted from [4], and the calculated mixing lines are from this study. The MgO, Cr, δ26Mg values for subducting Mg-rich carbonates are 13%, 10×10-6, and -2.0‰, respectively (after [12]), for mantle peridotites are 45%, 2200×10-6, and -0.25‰, respectively (after [20-21]), and for silicate-rich melts from subducted oceanic crust are 2.5%, 26×10-6, and 0.25‰, respectively (after [22]).

图4 含水地幔和含水+CO2地幔的固相线的压力-温度图(据文献[30]修改)

Fig.4 Pressure-temperature (P-T) diagram showing the solidus for hydrated mantle and hydrated mantle with CO2. Modified after [30].

图5 东亚大地幔楔软流圈地幔大尺度轻Mg同位素异常与地幔过渡带600 km地震波层析成像高速体(滞留俯冲板片)分布区的空间重合(资料据文献[1,6,32]修改)

Fig.5 Spatial overlay. The area with large-scale light Mg isotope anomaly in the asthenospheric mantle in the big mantle wedge beneath East Asia (left) overlaps with high velocity anomaly (with stagnant plates) imaged by seismic tomography at 600 km depth in the mantle transition zone (middle, right). Modified from [1,6,32].

图6 橄榄岩及石榴子石辉石岩(相当深俯冲洋壳)和碳酸盐化橄榄岩及碳酸盐化榴辉岩(相当碳酸盐化深俯冲洋壳)的固相线T-P图及含CO2超碱性霞石岩类熔体产生的T-P条件(据文献[13]修改)。该图显示对流上地幔的“洋中脊绝热线”与橄榄岩+CO2固相线相交于约300 km深度。因此,地幔过渡带上覆的碳酸盐化对流上地幔(深度约400 km)仅在上涌至300 km才开始发生部分熔融。

Fig.6 P-T diagram showing the solidus for peridotite, garnet pyroxenite (representing deep subducting oceanic crust), carbonated peridotite, and carbonated eclogite (representing deep subducting carbonated oceanic crust) and P-T conditions for the formation of CO2-bearing, strongly alkaline nephelinitic melt. The convective upper mantle mid-ocean ridge adiabatic line intersects the solidus for peridotite + CO2 at ~300-km depth, therefore, the carbonated convective upper mantle (at ~400 km depth) overlying the mantle transition zone starts to undergo partial melting only after upwelling to ~300 km depth.

图7 菱镁矿、金刚石、初始参考地球模型(PREM)和橄榄岩熔体的密度随深度的变化(据文献[36]修改)

Fig.7 Pressure and density depth profiles for magnesite, diamond, the Preliminary Reference Earth model (PREM), and peridotite melt. Modified from [36].

图8 大地幔楔地幔对流示意图(据文献[37]修改)

Fig.8 Mantle convection in a big mantle wedge. Modified after [37].

图9 (a)中国东部大陆区年龄小于110 Ma的玄武岩年龄统计直方图(据文献[6]);(b)中生代以来不同时代全球大陆裂谷长度统计图,图中L1和L2是两种统计方法的结果(详见文献[38])

Fig.9 Statistical evidence. (a) A compilation of ages (<110 Ma) of basalts from eastern China (adapted from [6]). (b) A compilation of global continental rift lengths since the Mesozoic using two different statistical methods (adapted from [38]).

图10 地球大气两次氧增高事件示意图(据文献[41]修改)

Fig.10 Two great oxidation events on Earth. Modified after [41].

图11 自早白垩世以来,海底水温(a)和大气CO2分压(b)的演化(据文献[42]修改)

Fig.11 Evolution of (a) seafloor water temperature and (b) atmospheric CO2 partial pressure since the Early Cretaceous. Modified after [42].

图12 地幔碳的板块输入和输出通量估计,碳输出的蓝色标记与俯冲板块输入碳有关(据文献[43]修改)。本文指出板内玄武岩火山可分为“大地幔楔”和“地幔柱”两类,大地幔楔板内火山释放的碳与俯冲板片输入地幔的碳有关。

Fig.12 Estimates of carbon inputs/outputs to the mantle. Carbon output from island arc (blue box) is related to carbon inputs to subduction zones (modified from [43]). According this study, intraplate basaltic volcanoes can be divided into “big mantle wedge” and “mantle plume” types. Carbon released from volcanoes in a big mantle wedge is related to carbon inputs from the subducting plate.